1. Field of the Invention
The invention relates generally to the field of obtaining nuclear magnetic resonance (NMR) measurements from rock formations. More specifically, the invention relates to methods for applying improved speed correction to echo data received by an NMR tool.
2. Background Art
Nuclear magnetic resonance (NMR) can be used to determine various characteristics of subsurface formations and/or samples. NMR logging tools can be used downhole to obtain these characteristics, which then can be used to assist in the determination of, for example, the presence, absence, and/or location of hydrocarbons in a given formation or sample.
Conventional NMR logging, well known in the art, generally involves deploying in a wellbore an NMR instrument, which uses magnetic fields to generate and detect various RF signals from nuclei in a formation or sample. Certain exemplary NMR techniques are described in U.S. Pat. No. 6,232,778 assigned to Schlumberger Technology Corp., the entire disclosure of which is hereby incorporated by reference.
NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field B0 to the formation, such as through the use of permanent magnets. This field causes the proton spins to align in a direction parallel to the applied field (this step, which is sometimes referred to as the creation of longitudinal magnetization, results in the nuclei being “polarized”). Polarization does not occur immediately, but instead grows in accordance with a time constant T1, and may take as long as several seconds to occur. After sufficient time, a thermal equilibrium polarization parallel to B0 has been established.
Next, a series of radio frequency (RF) pulses are produced so that an oscillating magnetic field, B1, is applied. The first RF pulse (referred to as the 90-degree pulse) must be strong enough to rotate the magnetization from B0 substantially into the transverse plane (i.e., transverse magnetization). Additional RF pulses (often referred to as 180-degree pulses) are applied to create a series of spin echoes. The frequency of the RF pulses is chosen to excite specific nuclear spins of a particular region of the sample that is being investigated.
Two time constants are associated with the relaxation processes of the longitudinal and transverse magnetization: T1 and T2. The spin-lattice relaxation time (T1) is the time constant for longitudinal magnetization to return to its thermal equilibrium value in the static magnetic field. The spin-spin relaxation time (T2) is the time constant for the transverse magnetization to return to its thermal equilibrium value which is zero.
The spin echoes (also known as “echoes” or “echo data”) collected by conventional NMR logging tools are normally inverted and then displayed in relaxation or T2 space. Various conventional methods exist for inverting spin echoes to be displayed in T2 space, such as those described in Freedman, R. and Morriss, C. E.: Processing of Data From an NMR Logging Tool, SPE 30560 (October 1995).
One issue arising in conventional NMR logging tools is that the movement of the tool in the downhole environment affects the T2 relaxation time reported by the tool. FIGS. 1A-1C are diagrams showing a conventional NMR logging tool 120 disposed within a borehole 11 or wellbore at three different time periods. The tool 120 shown in FIG. 1 is in the process of making measurements and upwardly-traversing the borehole 11 in the formation 106. As can be seen in FIG. 1, the NMR tool 120 includes a magnet 108 used for creating the magnetic field that can cause transverse magnetization in a given region 110. As the tool 120 moves upward, the region 110 correspondingly moves upward. Accordingly, by the time the tool 120 has moved from the first time period shown in FIG. 1A to the last time period shown in FIG. 1C, the region 110 over which the tool 120 is applying the magnetic fields and detecting the corresponding response has shifted. This motion of the instrument affects the T2 relaxation time reported by the NMR tool 120, as some of the polarized material moves out of the sample region 110. This issue is often referred to as a “speed effect.”
The speed effect occurring in conventional NMR tools can reduce the amplitude of the echoes as a function of time. This can cause the reported T2 distribution to be artificially shifted to slightly earlier times. The speed effect is more pronounced as the speed of the tool's 120 movement up the borehole 11 is increased. FIG. 2 is a graph depicting the speed effect of a conventional NMR logging tool 120 at three different speeds. The same interval was logged using different cable speeds—at 250 feet per hour 112, 1000 feet per hour 114, and 1800 feet per hour 116—for the data shown in FIG. 2. The shift to earlier decay time at faster logging speeds is easily seen.
Attempts have been made to correct for the speed effect in NMR measurements. Conventional speed correction methods include applying multiplicatively a correction factor that varies over time to the echoes received by the tool 120. Depending on how the correction factor is calculated, the multiplicative application of the correction factor can include dividing the echo data received by the correction factor or multiplying the echo data by the correction factor. FIG. 3 includes three graphs illustrating the effect of applying a conventional speed correction method. The first graph 118 in FIG. 3 shows the measured echoes 124 and a Single Value Decomposition (SVD) 126 fit of those echoes. The SVD will be discussed in more detail below. As can be seen in this graph 118, there is some noise in the echo data 124, as some of the echoes 124 periodically vary greatly (i.e., beyond a standard deviation 128) from the fit line.
The second graph 121 in FIG. 3 shows a speed correction factor 131 that varies over time that can be applied to the echo data 124. Determining the correction factor 131 depends on the speed of the measurement, the time taken for the measurement (e.g. number of echoes and echo spacing), the polarization/T1 times of the protons/nuclei, and the details of the tool 120 design, as is recognized by one of ordinary skill in the art.
The third graph 122 in FIG. 3 shows the echo data 132 and SVD line 126 with the speed correction factor 131 applied. Dividing the echo data 124 by this factor 131 corrects for the speed effect, by shifting the signal decay later in time and thereby compensating for the artificial shortening caused by the speed effect. However, conventional methods such as this have the unwanted effect of also increasing the noise inherent in the signal.
Accordingly, there is a need in the art for methods and systems for applying speed correction that overcome one or more of the deficiencies that exist with conventional methods.